1. Create a business use case diagram. I’m not suggesting that you begin to write business use cases as this would probably require more time and effort than you currently have. But by creating a business use case diagram you are creating a clearly defined structure for future requirements gathering tasks.
2. Prioritize your business use cases in order of importance. Those use cases that are most important to your business should take higher priority.
3. Identify subject matter experts (SMEs) per business use case.  No one person or group of people will usually be suitable for gathering requirements for all use cases.
4. Determine which methods of requirements gathering make the most sense based on your business environment, project environment, etc. (requirement workshops such as JAD sessions, one-on-one interviews, questionaires, brainstorming, etc.)
5. Prior to eliciting requirements determine what attributes you would like to capture for each requirements (i.e., source of requirement, complexity, priority, stability, etc)
6. Elicit your functional and non-functional requirements per business use case. This groups requirements that have the same business goal together which is your first step at organizing your requirements efficiently
7. Create a traceability matrix. By tracking the relationship between business use cases and system requirements, functional requirements and features, etc., you will be able to find areas that may need additional work (more requirements gathering)

Hi Sonavi,

This should help you start with the use cases:

http://en.wikipedia.org/wiki/Use_case

http://alistair.cockburn.us/index.php/Use_case_fundamentals

And this one is more of an UML overview  http://atlas.kennesaw.edu/~dbraun/csis4650/A&D/UML_tutorial/use_case.htm but it presents the types of diagrams you could use to supplement the use case text.

Vessela

Sonavi,

I have read a few books on use cases.  The following would be my recommendation. It is a very well written and well structure book.  It keeps a fairly practical approach but gives a good overview of use cases as a whole.

Use Case Modeling By Kurt Bittner, Ian Spence

You should be able to follow the link provided to find it along with other great books in the Modern Analyst Bookstore.